1. Field of the Invention
The invention herein relates to desalination of saline water. More particularly it relates to processes for desalination of desalination, especially of sea water, for production of fresh water.
2. Description of the Prior Art
Many countries have considered desalination of saline water, especially sea water, as a source of fresh water for their arid coastal regions or for regions where water sources are brackish or have excessive hardness. Typical areas where desalination has been considered or is in use include southern California in the United States, Saudi Arabia and other Middle Eastern countries, Mediterranean countries, Mexico and the Pacific coast countries of South America. Similarly, islands with limited fresh water supplies, such as Malta, the Canary Islands and the Caribbean islands, also use or have considered desalination of sea water as a fresh water source.
Desalination process in the past, however, have had high energy requirements per unit of desalinated water product and have operated at relatively low yields, typically 35% or less based on feed. They have therefore been economical only for those locations where fresh water shortages are acute and energy costs are low. While desalination plants have also been used in other areas, those uses have generally been in times of drought or as stand-by or supplemental sources of fresh water when other sources are temporarily limited or unavailable, since in most such locations current desalination processes cannot compete effectively with other sources of fresh water, such as overland pipelines or aqueducts from distant rivers and reservoirs.
However, because there is a vast volume of water present in the oceans and seas, and because direct sources of fresh water (such as inland rivers, lakes and underground aquifers) are becoming depleted, contaminated, or reaching capacity limits, there is a extensive research underway through the world for an
economical process for desalination of saline water, and especially of sea water.
Desalination of sea water must take into account important properties of the sea water: turbidity, hardness and salinity (ionic content and total dissolved solids [TDS]) and the presence of suspended particulates and microorganisms. These properties place limits of 30%-35% on the amount of fresh water yield that can be expected from prior art desalination process as used or proposed. Reference is made in this application to xe2x80x9csalinexe2x80x9d water, which includes sea water from seas and oceans and water from various salt lakes and ponds, brackish water sources, brines, and other surface and subterranean sources of water having ionic contents which classify them as xe2x80x9csaline.xe2x80x9d This can generally be considered to be water with a salt content of xe2x89xa71000 parts per million (ppm); Kirk-Othmer (ed.), CONCISE ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, 1252-1254 (1985). Since of course sea water has the greatest potential as a source of potable water (i.e., generally considered to be water with a salt content of xe2x89xa6500 ppm [ibid.]), this application will focus on sea water desalination. However, it will be understood that all sources of saline water are to be considered to be within the present invention, and that focus on sea water is for brevity and not to be considered to be limiting.
Multistage flash distillation (MSFD) is the major desalination process used worldwide. Alone, it accounts for about 48% of total world desalination capacity as compared to 36% produced by the reverse osmosis (RO) process. The rest (16%) is produced by a variety of processes, primarily electrodialysis (ED), multiple effect distillation (MED) and vapor compression distillation (VCD). Saudi Arabia is the leading user of MSFD and the United States is the largest user of the RO process. MSFD, MED and VCD processes are used exclusively in sea water desalination, while ED is applied in brackish water desalination and pure water preparation. The RO process is applied to both sea water and brackish water feed but in the past its application was primarily in brackish water, drinking water and in pure water preparation. More recently, however, sea water RO (SWRO) desalination has become more common, utilizing relatively large plants of 10-15 million gallon/day (mgd) [39-57 million liter/day (mLd)] plants.
SWRO plants are severely limited by factors such as turbidity (TDS) of the water feed. The feed osmotic pressure increases with the TDS. From the principles of RO the applied pressure is necessarily used to overcome the osmotic pressure and the remaining pressure is the net water driving pressure through the membrane. The lower the osmotic pressure can be made, the greater the net water driving pressure, and therefore the greater the amount of pressure available to drive the permeate water through the membrane, which also produces a higher quantity of product.
Various types of filtration or coagulation-filtration systems have been used for treatment of water and other liquid solutions and suspensions for removal of particulate matter. For removal of fine particles with sizes less than 1 xcexcm, microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and hyperfiltration/reverse osmosis (HFRO) membrane filtration are employed. MF is used with particles having sizes in the range of 0.08-2.0 xcexcm, the UF membrane process is more effective for finer particles having sizes in the range of 0.01-0.2 xcexcm and of molecular weight (MW) in the range of 10,000 g/mole and above. Both the MF and UF membrane processes are true filtration processes where particle separation is done according to size. Moreover, each of the MF and UF membranes has its own characteristic pore size and separation limits. These filtration processes differ significantly from the RO process which is a differential pressure process for separation of ionic particles with sizes of 0.001 xcexcm or less and molecular weights of 200 g/mole or less.
The NF membrane process falls in-between the RO and UF separation range, and is suited for the separation of particle sizes in the range of 0.01-0.001 xcexcm and molecular weights of 200 g/mole and above. Unlike either UF or RO, however, NF acts by two principles: rejection of neutral particles according to size and rejection of ionic matter by electrostatic interaction with a negatively charged membrane; Rautenbach et al., Desalination, 77:73-84 (1990). NF has been used in Florida for treatment of hard water to produce water of drinking water standards. NF has also been used for removal of color, turbidly, and dissolved organics from drinking water; Duran et al., Desalination, 102:27-34 (1995) and Fu et al., Desalination, 102: 47-56 (1995). NF has been used in other applications to treat salt solution and landfill leachate; Linde et al., Desalination, 103:223-232 (1995); removal of sulfate from sea water to be injected in off-shore oil well reservoirs; Ikeda et al., Desalination, 68:109 (1988); Aksia Serch Baker, Filtration and Separation (June, 1997).
It would therefore be of substantial worldwide interest to have available a process which would economically produce a good yield of fresh water from saline water, especially from sea water, and which would effectively deal with the problems mentioned above; i.e., removal of hardness and turbidity from such saline water and the lowering of total dissolved solids.
I have now invented a process which, by combining two or more substantially different water treatment processes in a manner not heretofore done, desalinates saline water, with particular emphasis on sea water, to produce a very high yield of high quality fresh water, including potable water, at an energy consumption per unit of product equivalent to or better than much less efficient prior art desalination processes. In my process nanofiltration as a first desalination step is synergistically combined with a multistage flash distillation, multieffect distillation, vapor compression distillation or sea water reverse osmosis process to provide an integrated system by which saline water (especially sea water) can be efficiently and economically converted to high quality fresh water in yields which are significantly larger by 70%-80% than the yields available from the prior art processes, alone or in combinations heretofore known or described. Thus, while individual steps have been separately known and such steps have individually been disclosed in combination with other processes for different purposes, the present process has not previously been known to or considered by those skilled in the art, and nothing in the prior art has suggested the surprising and unique magnitude of improvement in saline water desalination obtained through this process as compared to prior art processes.
Therefore, in a broad embodiment, the invention is of a desalination process which comprises passing saline water containing hardness scale forming ionic species, microorganisms, particulate matter and/or high total dissolved solids through nanofiltration to form a first water product having reduced content of said ionic species, microorganisms or particulate matter and lowering TDS, and thereafter passing said first water product through sea water reverse osmosis, multistage flash distillation, multieffect distillation or vapor compression distillation to form a second water product also having reduced salinity.
In another broad embodiment, the invention involves a desalination process as in claim 1 which comprises passing said saline water containing hardness-generating ionic species, microorganisms or particulate matter through nanofiltration to form a first water product having reduced content of said ionic species, microorganisms or particulate matter, thereafter passing said first water product through sea water reverse osmosis to form a second water product also having reduced salinity and a reject product having increased salinity, and thereafter passing said reject product through multistage flash distillation to produce a third water product having salinity less than that of said reject product.
The process readily and economically yields significant reductions in saline water (especially sea water) properties, and produces good fresh water, including potable water. Typically a process of this invention will produce, with respect to the sea water feed properties, calcium and magnesium cation content reductions on the order of 75%-95%, total salinity reductions on the order of 25%-38%, pH decreases of about 0.4-0.5, and total dissolved solids content (TDS) reductions of about 35%-50%.